1. Field of the Invention
The present invention relates to an electroluminescent device (EL device).
2. Background Art
In electroluminescent devices (EL devices), which use electroluminescence, attention is now focused on their use as light-emitting devices in various types of displays, and so forth. EL devices are self-light-emitting devices of injection luminescence type, which use luminescence that occurs at the instance electrons and holes arriving at a luminescent layer recombine with each other. The basic structure of EL devices is that a luminescent layer containing a luminescent material is situated between a cathode and an anode. EL devices are classified into inorganic ones using inorganic compounds as the luminescent material and organic ones using organic compounds as the luminescent material.
Recently, electroluminescent devices using, as the luminescent material, quantum dots have also been proposed (e.g., Japanese Laid-Open Patent Application No. 2005-38634, Published Japanese Translations No. 2005-502176 and No. 2006-520077 of PCT International Publications for Patent Applications, and Seth Coe et al., Nature 420, 800-803 (2002)). Quantum dots are nanometer-sized fine particles of a semiconductor (semiconductor nanocrystals). Owing to their quantum confinement effect (quantum size effect) with which electrons and excitons are confined in nanometer-sized small crystals, quantum dots exhibit characteristic optical and electrical properties, and their utilization is expected in a wide variety of technical fields. A quantum dot emits light having a wavelength dependent on its particle diameter, so that it is possible to obtain lights different in wavelength by controlling the particle diameter. Further, since light emitted by a quantum dot is narrow in spectral width, it is excellent in color purity.
Although a layer containing quantum dots can be formed by a wet process which dispersion of quantum dots is applied, or a dry process which a material for quantum dots is deposited to form a film by such a technique as vapor deposition or sputtering, there is a tendency to adopt a wet process from the viewpoint of simplicity of apparatus and process, smoothness of the layer formed, and so forth.
However, using a wet process to form a quantum dots-containing layer is disadvantageous in that quantum dots easily coagulate in their dispersion. For the purpose of controlling the dispersibility of quantum dots in a liquid and also the particle diameter of quantum dots in their production, the surfaces of quantum dots are protected by a protective material. Typical examples of protective materials effective in controlling the particle diameter of quantum dots in their production and in improving the dispersibility of quantum dots in a liquid include trioctylphosphine oxide (TOPO: [CH3(CH2)7]3PO).
Published Japanese Translation No. 2005-502176 of PCT International Publication, for example, is one of the documents that deal with techniques concerning materials for protecting quantum dots. This document describes an electroluminescent device comprising a hole-control means for injecting and transporting holes, a luminescent layer that is in contact with the hole-control means and that contains quantum dots, each dot having on its surface at least one capping molecule having a functional unit that causes injection of excitons to the quantum dot, and an electron-control means for injecting and transporting electrons to the luminescent layer, being in contact with the luminescent layer. In this document, molecules having electron-transporting, hole-transporting, or exciton-transporting parts are mentioned as the capping molecule.
In Seth Coe et al., Nature 420, 800-803 (2002), recombination of electrons and holes is caused in a monolayer of quantum dots to make the quantum dots emit light. In this technique, since the monolayer serves as a luminescent layer, the luminescent area along the film thickness is small, which limits the opportunity for recombination of electrons and holes. Moreover, since such a monolayer of quantum dots is formed by phase separation, it is disadvantageous from the viewpoint of interlaminar adhesion, driving stability, and thermal stability. Another problem is that since molecular defects caused in the monolayer lead directly to emission defects, it is difficult to control the conditions for film deposition.
Besides, protective materials, represented by TOPO, whose main purpose is to ensure the dispersibility of quantum dots are poor in charge-transport characteristics; they barely perform the function of injecting electrons and/or holes to quantum dots and hardly serve as a space where electrons and holes recombine with each other.
The aforementioned publication No. 2005-502176 describes the following two mechanisms of emission of light by an EL device. One mechanism is as follows: electrons and/or holes are transferred to the quantum dots by the electron-transporting parts and/or the hole-transporting parts of the capping molecules present on the surfaces of the quantum dots, and these charges recombine with each other in the quantum dots to emit light. Another mechanism is as follows: electrons and holes recombine with each other in the luminescent layer, that is, in a part other than the quantum dots, to produce excitons, and by the exciton-transporting parts of the capping molecules existing on the surfaces of the quantum dots, the excitons are transferred from the luminescent layer to the quantum dots to emit light.
In the EL device described in this publication No. 2005-502176, if the transferability of excitons to the capping molecules having the exciton-transporting parts is not ensured, recombination of electrons and holes in the luminescent layer may cause deterioration in luminous efficiency of the device.